INTERANNUAL AND INTRASEASONAL VARIABILITY OF SEA-ICE EXTENT IN THE SEA OF OKHOTSK (1979–2024)

Abstract

This paper presents the results of an analysis of interannual and intraseasonal variability of sea-ice extent in the Sea of Okhotsk for 1979–2024 based on pentad sea-ice charts from the Japan Meteorological Agency processed with the Ice Data Processing software. Sea-ice extent was calculated as the fraction of the sea area covered by ice and referenced to the 1991–2020 climatological baseline. Regression and spectral analyses were applied to time series of seasonal mean (December–May) and seasonal maximum sea-ice extent; pentad rates of change were also evaluated to quantify intraseasonal dynamics and timing of transitions between ice-growth and ice-decay phases. A statistically significant decline of seasonal mean sea-ice extent by 16 % over the 45-year period (–3.5 % per decade) and a stronger decrease of seasonal maxima (–4.0 % per decade) are demonstrated, with a high correlation between mean and maximum values (r = 0.89). Two contrasting regimes are identified: higher ice conditions in 1979–2004 (44.4 %) and lower ice conditions in 2005–2024 (34.8 %), along with quasi-periodic variability on time scales of 3–7 years and a low-frequency component of 15–23 years. Intraseasonal analysis reveals weakened ice formation in early winter (a decrease in the mean growth rate in December–January from 4.3 % to 3.7 % per pentad), an advance of the transition to the ice-decay phase by about 10 days (from 15–20 March to 5–10 March), and reduced amplitudes with a shift of peak growth and melt rates (the growth peak decreased from 5.8 % on 25–31 December to 5.2 % on 15–20 January; the strongest ice loss shifted from 15–20 April to 5–10 April with a lower magnitude). The obtained results quantitatively characterize the ongoing weakening and shortening of the ice season in the Sea of Okhotsk and can be used for climate monitoring and validation of regional seasonal sea-ice models.